When the human body becomes infected with particularly severe forms of diseases like tuberculosis and leprosy, its immune response is to produce lots of interferon-beta, a virus-destroying protein. There's just one problem. Those diseases are bacteria, not viruses.

Researchers at the University of California Los Angeles first discovered this unusual property while studying leprosy. Patients whose immune systems were dealing with a milder form of the disease produced interferon-gamma, which is the correct protein to ward off bacterial infections. But when the leprosy was more severe, interferon-beta became more prominent. Not only is this antiviral protein useless in fighting off bacterial infection, it also can jam the proper working of interferon-gamma, meaning the bacteria has free rein to spread and multiply within the body.

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So why does this faulty immune response occur? The researchers looked at the recent outbreaks of homeless people in the LA area, which typically occur most often in the early spring. Flu outbreaks can be relatively common in shelters, where the homeless typically live in very close quarters. These outbreaks, combined with vitamin D deficiencies due to lack of exposure to sunlight, can combine to provoke a strong antiviral response in the immune system, which means producing lots of interferon-beta.

This in turn prevents the immune system's usual production of interferon-gamma, leaving the body exposed and susceptible to bacterial infection. This effect might not be observed until months after flu season passes, which would line up with the observed spring outbreaks of tuberculosis. For more on this story, check out the original paper at Science and this report from BBC News.